This research is designed to further our understanding of the mechanism involved in the control of glomerular filtration rate and renal blood flow. The macula densa feedback theory of autoregulation will be examined in healthy dogs, primarily because this animal has been the subject of the most intense investigation concerning the phenomena of renal autoregulation which is due in no small part to the ease with which renal blood flow can be measured with an elctromagnetic flow meter. Calcium is the ion upon which these efforts will be focused because alterations in serum calcium concentration have been inversely related to changes in renal hemodynamics. Micropuncture and microperfusion studies will be conducted to evaluate the response of single nephron glomerular filtration rate to wide fluctuations in calcium delivery to the distal nephron. Overall the results obtained should demonstrate the nature of the relationship between calcium delivery to the distal nephron and the rate of nephron filtration - a relationship which additively should clarify our understanding of the regulation of the rate of filtration by the whole kidney. Combined clearance and micropuncture studies in parathyroidectomized dogs and in other dogs whose vitamin D intake has been manipulated should provide some evidence of the role of the kidney in maintaining calcium homeostasis, during disruptions in serum calcium concentration.